In high speed and low noise device applications, the focus has been on designing and fabricating high electron mobility transistors (HEMTs) or modulation-doped field effect transistors (MODFETs) where carrier (e.g. electrons, holes) conduction occurs in an undoped channel layer such that the carrier mobility is not limited by impurity scattering and high carrier mobility is achieved. In general, these high speed electronic devices are often used as low-noise amplifiers, power amplifiers, satellite receivers and transmitters operating in the microwave and rf regime, and the material of choice is usually the faster but more expensive III-V materials system and technology such as GaAs and InP. A complicated and costly III-V materials technology is not very desirable in the semiconductor industry whereas a less-expensive SiGe materials system which is fully compatible with present Si technology is more desirable and far easier to integrate with existing Si-CMOS device technology.
One example of a material system compatible with Si technology is described in U.S. Pat. No. 5,019,882 which issued on May 28, 1991 to P. M. Solomon entitled “Germanium Channel Silicon MOSFET” and assigned to the assignee herein. In U.S. Pat. No. 5,019,882, a channel having improved carrier mobility comprises an alloy layer of silicon and germanium which is grown above a silicon substrate. The alloy layer is kept thin enough for proper pseudomorphic dislocation free growth to occur. A layer of silicon is formed over the alloy layer and is oxidized partially through to form a dielectric layer. A gate region is formed over the silicon dioxide.
A second example of a high performance SiGe device structure compatible with Si technology, is described in U.S. Pat. No. 5,534,713 which issued on Jul. 9, 1996 to K. E. Ismail entitled “Complementary Metal-Oxide Semiconductor Transistor Logic Using Strained Si/SiGe Heterostructure Layers” and assigned to the assignee herein. In U.S. Pat. No. 5,534,713 a silicon CMOS transistor structure is described utilizing a buried SiGe channel under compressive strain with enhanced hole mobility for a p-channel device, and a buried Si channel under tensile strain with enhanced electron mobility for an n-channel device fabricated on a strained Si/SiGe heterostructure design. Further in U.S. Pat. No. 5,534,713 the proposed compressively-strained SiGe layer serving as a p-channel for the p-channel field effect transistor is described as having a composition of germanium in the range from 50 to 100% and with a preferred composition of 80%. Thus far, prototype SiGe p-channel MODFETs utilizing this channel design and composition at the Thomas J Research Center, IBM Corporation have yielded hole mobilities only up to 1,000 cm2/Vs at room temperature.
The compatibility and fabrication of a Ge-channel MODFET using existing Si technology has been demonstrated by molecular beam epitaxy (MBE) techniques where modulation-doped FET structures with hole channels consisting of a pure Ge layer were grown by molecular beam epitaxy on a Si substrate. In particular, room temperature hole mobility for a two-dimensional hole gas (2DHG) in a modulation-doped, strained Ge layer (grown by MBE) has been reported as high as 1,870 cm2/Vs in a publication by G. Höck, T. Hackbarth, U. Erben, E. Kohn and U. König entitled “High performance 0.25 μm p-type Ge/SiGe MODFETs”, Electron. Lett. 34 (19), 17 Sep. 1998, pp 1888-1889 which is incorporated herein by reference. In G. Höck et al., for the 0.25 μm gate length devices, the p-type Ge channel MODFETs exhibited a maximum DC extrinsic transconductances of 160 mS/mm while the maximum drain saturation current reached up to a high value of 300 mA/mm. For the RF performance, a unity current gain cutoff frequency ƒT of 32 GHz and a maximum frequency oscillation ƒmax of 85 GHz were obtained.
There is a growing interest in designing and fabricating high speed low temperature MOSFETs and bipolar transistors for high speed cryogenic applications such as read out electronics for cooled infrared detectors, fast processors, and low noise amplifiers. To this end, a Ge channel device structure which can be operated in the temperature range from room temperature (300 K) down to cyrogenic temperature (<T=77 K) while having even higher transport characteristic is the ideal solution. An example of a modulation-doped SiGe/Ge heterostructures with a 2D hole channel consisting of pure Ge which is operable at both room temperature and at 77 K has been reported in a publication by “U. König and F. Schaffler entitled “p-Type Ge-Channel MODFET's with High Transconductance Grown on Si Substrates”, Electron. Dev. Lett. 14 (4), 4 Apr. 1993, pp 205-207 which is incorporated herein by reference.
Another example of a field effect transistor having a high carrier mobility suitable for high speed and low temperature operation is described in U.S. Pat. No. 5,241,197 which issued on Aug. 31, 1993 to E. Murakami et al entitled “Transistor Provided with Strained Germanium Layer”. In U.S. Pat. No. 5,241,197, a strain control layer grown by molecular beam epitaxy is provided beneath a germanium layer to impose a compressive strain on the germanium layer. The composition of the strain control layer is used to generate the compressive strain. The carrier mobility in the strained germanium layer is reported to be 3000 cm2/Vs. However, no measurements or data have been subsequently published of Ge properties or Ge layered structures with mobilities over 2000 cm2/Vs at room temperature. Reported values of hole mobilities of Ge layered structures at room temperature of 1900 cm2/Vs are found on page 315 and specifically in Table 8.1 of D. W. Greve, Field Effect Devices and Applications published in 1998 by Prentice-Hall, Inc. Upper Saddle River, N.J.